Distillation of high boiling esters



Ju1y13, 1943.y J'. E. JEWETT 2,324,088 l DISTILLATION OF HIGH BOILINGESTERS Filed April 16, 1940 I INVENTOR HMP/f @asf/H dfn/77 BY f,

A TTORNEYN Patented July 13, 1943 AUNITED ,STATESY PATENT OFFICE.

DISTILLATION OF HIGH BOILING ESTERS Joseph E. Jewett, Larchmont, N. Y.,assigner to American Cyanamd Company, New York, N. Y., a corporation ofMaine Application April 16, 1940, Serial No. 329,849

(Cl. 21R-'52) 1 Claim.

This invention relates to a process for distilling high-boiling organicesters, and more particularly to a process for the vacuum distillationof high-boiling esters of dicarboxylic acids.

During the distillation of high-boiling organic esters, decomposition islikely to occur as a result of heating the product to a high temperatureand maintaining it at the high temperature for a prolonged period oftime. To avoid this decomposition, it has been attempted in the past toapply vacuum .distillation methods to processes for distilling thesehigh-boiling esters.

'I'he most common method of vacuum distillation, which consists ofdistilling material under sufllciently reduced pressure to give therequired boiling point, is unsatisfactory due chiefly to overheating ofthe bottom portions offmaterial caused by the hydrostatic head developedin the still. Consequent decomposition is considerable when thedecomposition range lies close to the boiling point under the reducedpressure used. Another factor which greatly contributes to thedecomposition of the material undergoing distillation is the necessityof maintaining the entire charge at the distillation temperaturethroughout the entire distillation.

Another method frequently applied provides for introduction of materialinto a highly heated tube of small diameter under vacuum. Evaporationtakes place in the tube and the material is ejected therefrom at a highvelocity by expansion of the vapor formed. It is obvious that thismethod is designed to avoid maintenance of the entire distilland at thedistillation temperature throughout the operation. .However, there aretwo inherent causes of overheating present in this method. First, if thelength of the tube (as determined by experiment) is not just exactlyright, there will beoverheating of the liquid and/Orsuperheating of thevapor. Second, uid friction in the tube will cause a pressure drop whichwill, in turn. allow overheating at the feed end since the boiling pointalong the length of the tube varies with pressure.

lAccording to the present invention, I have found that high-boilingorganic esters may be distilled with little, if any, consequentdecomposition if an excess of the material isiirst heated to atemperature just below the decomposition temperature and then flashed ina low pressure chamber. Undistilled material is returned, through acooler if desired, to the charge and recirculated.

It is believed that the process of the present invention can best beunderstood by reference to the accompanying drawing in which Fig. 1 is adiagrammatic illustration of an apparatus suitable for distillinghigh-boiling esters of dicarboxylic acids according to the process ov.-lined above.

The charge of ester to be distilled is placed in vtank I and from thereis delivered to heater 6 through seal loop 5, by meansof a circulatingpump 3 which is driven by motor 4. The heater 6 is a tubular heater ofconventional design heated by a medium which is introduced through pipe21 and leaves through pipe 3|.

After the material is heated to a temperature below that at whichvaporization tak'es place, which temperature is regulated by thepressure existing on the system, the rate of flow, and the controller I8which will be described in detail later, it passes through pipe 3l' intoilash chamber 8 Where it strikes plate 38 and is spread in a sheet orspray, thus affording a large area for flash evaporation. Enough vaporis evolved to cool the liquid to its boiling point under the reducedpressure existng'in flash chamber 8' and the cooled material is returnedthrough pipe. 35, cooler 28, the cooling medium for which enters throughpipe 29 and leaves through pipe 30, and pipe 3B to seal chamber 2 fromwhich it# overows into tank I.

Vapors from chamber 8 pass through pipe I I to condenser I 2 which istted with cooling tubes I2a, the cooling medium for which enters throughpipe I3 and leaves through pipe I4. The condensed liquid then flowsthrough pipe 32 and seals 33 and 33a into storage tanks 34 and 34a. Anyuncondensed vapors leave the condenser I2 through pipe I5 to vacuum pumpI6, from which they are ejected through pipe I'I.

If the material being. distilledis not particularly heat sensitive andnot easily decomposed, the cooler 28 may be eliminated, the unvaporizedmaterial then returning directly from the ash chamber 8 to tank I. Thisfeature of the apparatus is entirely optional and may 'or may not beused, depending on the material.

Flash chamber 8 may be tted with fractionating plates 9 and reuxcondenser coil I0 if a.l

' seconds.

on two sides of orifice 39. The boiling point of the material underreduced pressure in the flash chamber v8 is determined, and thistemperature applied to one bulb I9 of a dierential temperaturecontroller I8 which holds a Xed temperature difference between bulbs I9and 20 by throttling the heating medium passing through controller valve2|.

Increase of pressure in theash chamber may cause the temperaturedeterminedby the absolute pressure in chamber 8 plus the setdifferential, which sum is the temperature on bulb 20, to go above thepredetermined safe limit. In order to avoid consequent decomposition ofmaterial being distilled, the same temperature as on bulb is applied tobulb 23 of an electrically operated limit controller 22 which has beenset at the appropriate maximum point.Y The contacts of Y this controller22 are opened, the coil of solenoid valve 24 is de-energized, the valvecloses and air prevent reverse operation of valve 25 unti1 it can bereset by hand.

The above-described conventional automatic temperature controller may beomitted and if desired, the control may be manual by reference toappropriate thermometers.

The present process, since it involves a flash distillation, is morerapid than the formerly used vacuum distillation methods and thisfeature is an advantage of my invention.

It is another advantage of the present invention that there is nooverheating of rmaterial or superheating of vapor. Only a small portionof the substance is heated to distillation temperature at one time andit is maintained at that tem perature for-only a very short time, i. e.,a few Thus, it will be apparent that decomposition of the material isavoided or appreciably decreased.

It is a further advantage of the invention that the use of an excess ofmaterial, coupled with the return of undistilled portionsforrecirculation. prevents the loss of any-substantial amount of material.It is thus unnecessary to maintain a high temperature for more than avery short time since undistilled material is automatically nm throughthe apparatus again.

Another advantage of the process of the present invention becomesparticularly apparent when the ester being distilled is one which isextremely easily decomposed. -In such cases, it is possible toarticially cool the undistilled material, which is already somewhat.cooled by the flash evaporation process, on its return to the tank. Inmany cases where the ester is notl particularly heat sensitive, thecooling attendantupon the flash evaporation itself is suillcient toprevent substantial decomposition of the undistilled portion of theester without any further cooling by artificial means. i

The process of the present invention described `above in considerabledetail is applicable to all dicarboxylic acids which comprises heatinga.

continuous, conned stream of the material to be distilled to atemperature below that at which decomposition takes place, rapidlyvaporizing the preheated material in a ash chamber maintained under avacuum, condensing the vaporized portion of the material, cooling theunvaporized portion sufficiently to prevent substantial decomposition ofthe ester, and returning the cooled unvaporized portion of the materialfor recirculation.

JOSEPH E. JEWE'I'I.

